A technique for forming a transistor by using a semiconductor over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. Silicon is known as a semiconductor applicable to a transistor.
As silicon which is used as a semiconductor of a transistor, either amorphous silicon or polycrystalline silicon is used depending on the purpose. For example, in the case of a transistor included in a large display device, amorphous silicon, which can be formed using an established technique for forming a film over a large-sized substrate, is preferably used. On the other hand, in the case of a transistor included in a high-performance display device where a driver circuit and a pixel circuit are formed over the same substrate, it is preferable to use polycrystalline silicon, which can be used to form a transistor having a high field-effect mobility. As a method for forming polycrystalline silicon, high-temperature heat treatment or laser light treatment which is performed on amorphous silicon has been known.
In recent years, an oxide semiconductor has attracted attention. For example, a transistor which includes an amorphous In—Ga—Zn oxide is disclosed (see Patent Document 1). An oxide semiconductor can be formed by a sputtering method or the like, and thus can be used for a semiconductor of a transistor in a large-sized display device. Because a transistor including an oxide semiconductor has high field-effect mobility, a high-performance display device in which, for example, a driver circuit and a pixel circuit are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including amorphous silicon can be retrofitted and utilized.
In 1985, synthesis of an In—Ga—Zn oxide crystal was reported (see Non-Patent Document 1). Further, in 1995, it was reported that an In—Ga—Zn oxide has a homologous structure and is represented by a composition formula InGaO3(ZnO)m (m is a natural number) (see Non-Patent Document 2).
In 2012, it was reported that a transistor including a crystalline In—Ga—Zn oxide has more excellent electrical characteristics and higher reliability than a transistor including an amorphous In—Ga—Zn oxide (see Non-Patent Document 3). Non-Patent Document 3 reports that a crystal boundary is not clearly observed in an In—Ga—Zn oxide including a c-axis aligned crystal (CAAC).
It is known that a transistor including an oxide semiconductor has an extremely low leakage current in an off state. For example, a CPU or the like with low-power consumption utilizing a characteristic of low leakage current of the transistor including an oxide semiconductor is disclosed (see Patent Document 2). Patent Document 3 discloses that a transistor having high field-effect mobility can be obtained by a well potential formed using an active layer formed of an oxide semiconductor.